This invention relates to nuclear magnetic resonance imaging methods. More specifically, this invention relates to a method for controlling image artifacts caused by substantially periodic NMR signal variations due, for example, to subject motion in the course of an NMR scan.
NMR has been developed to obtain images of anatomical features of human patients. Such images depict nuclear spin distribution (typically, protons associated with water and tissue), spin-lattice relaxation time T.sub.1, and/or spin-spin relaxation time T.sub.2 and are of medical diagnostic value. NMR data for constructing images can be collected using one of many available techniques, such as multiple angle projection reconstruction and Fourier transform (FT). Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more NMR experiments, each of which comprises at least an RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the resulting NMR signal. As is well-known, the NMR signal may be a free induction decay (FID) or, preferably, a spin-echo signal.
The preferred embodiments of the invention will be described in detail with reference to a variant of the well known FT technique, which is frequently referred to as "spinwarp". It will be recognized, however, that the method of the invention is not limited to FT imaging methods, but may be advantageously practiced in conjunction with other techniques, such as multiple angle projection reconstruction disclosed in U.S. Pat. No. 4,471,306, and another variant of the FT technique disclosed in U.S. Pat. No. 4,070,611. The spin-warp technique is discussed in an article entitled "Spin Warp NMR Imaging and Applications to Human Whole Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980).
Briefly, the spin-warp technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase-encoding gradient (G.sub.y) along that direction, and the observing a spin-echo signal in the presence of a second magnetic field gradient (G.sub.x) in a direction orthogonal to the phase-encoding direction. The G.sub.x gradient present during the spin-echo encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase-encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) monotonically in the sequence of views that are acquired to produce a set of NMR data from which an entire image can be reconstructed.
Object motion during the acquisition of NMR image data produces both blurring and "ghosts" in the phase-encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion, including cardiac and respiratory motion, each view of the NMR signal is acquired in a period short enough that the object may be considered stationary during the acquisition window. Blurring and ghosts, therefore, are due primarily to the inconsistent appearance of the object from view to view, and in particular, due to changes in the amplitude and/or phase of the NMR signal due to the motion.
Both blurring and ghosts can be reduced if the data acquisition is synchronized with the functional cycle of the object. This method is known as gated NMR scanning, and its objective is to acquire NMR data at the same point during successive functional cycles so that the object "looks" the same in each view. The drawback of gating is that NMR data may be acquired only during a small fraction of the object's functional cycle, and even when the shortest acceptable pulse sequence is employed, the gating technique can significantly lengthen the data acquisition time.
One proposed method for eliminating ghost artifacts is disclosed in U.S. Pat. No. 4,567,893, issued on Feb. 4, 1986, and is assigned to the same assignee as the present invention. In this prior application, it is recognized that the distance in the image between the ghosts and the object being imaged is maximized when the NMR pulse sequence repetition time is an odd multiple of one-fourth of the duration of the periodic signal variation (if two phase-alternated RF excitation pulses per view are used, as disclosed and claimed in commonly assigned U.S. Pat. No. 4,443,760, issued Apr. 17, 1984). It is recognized that this ratio can be used to alleviate ghosts due to respiratory motion. While this method, indeed, improves image quality, it does impose a constraint on the NMR pulse sequence repetition time and it often results in a longer total scan time. It also assumes that the motion is periodic. Its effectiveness is diminished when the subject's breathing is irregular because the ghosts are blurred and can overlap the image.
Another method for reducing the undesirable effects due to periodic signal variations is disclosed in U.S. Pat. No. 4,706,026 issued Nov. 10, 1987 and entitled "A Method For Reducing Image Artifacts Due To Periodic Variations In NMR Imaging." In one embodiment of this method, an assumption is made about the signal variation period (e.g. due, for example, to patient respiration) and the view order is altered from the usual monotonically increasing phase-encoding gradient to a preselected order. This involves establishing the order in which either the gradient parameters, i.e. the amplitude of the phase-encoding gradient pulses (in the spin-warp method) or the direction of the read-out gradient pulses (in the multiple angle projection reconstruction method) are implemented. For a given signal variation period, a view order is chosen so as to make the NMR signal variation as a function of the phase-encoding amplitude (or gradient direction) be at a desired frequency. In one embodiment, the view order is selected such that the variation period appears to be equal to the total NMR scan time (low frequency) so that the ghost artifacts are brought as close to the object being imaged as possible. In another embodiment (high frequency), the view order is chosen to make the variation period appear to be as short as possible so as to push the ghost artifacts as far from the object as possible.
This prior method is effective in reducing artifacts, and is in some respects ideal if the variation is rather regular and at a known frequency. On the other hand, the method is not very robust if the assumption made about the motion temporal period does not hold (e.g., because the patient's breathing pattern changes or is irregular). If this occurs, the method loses some of its effectiveness because the focusing of the ghosts, either as close to the object or as far from the object as possible, becomes blurred. A solution to this problem is disclosed in U.S. Pat. No. 4,663,591 entitled "A Method For Reducing Image Artifacts Due To Periodic Signal Variations in NMR Imaging." In this method, the non-monotonic view order is determined as the scan is executed and is responsive to changes in the period so as to produce a desired relationship (low frequency or high frequency) between the signal variations and the gradient parameter.
While the above described methods reduce motion artifacts, they do rely on some regularity, or predictability, in the cyclic motion which is not always present. For example, irregularities may occur during a breathing cycle which introduces errors into the data acquired for one or more views of the scan. One method for reducing such spurious errors is to perform two scans, or acquire twice the necessary data during a single scan and then average the information acquired for each view to produce an image of increased quality.